HVDC plants comprise converters which on their alternating current sides are coupled to alternating current networks via converter transformers, and on their direct current sides are coupled to each other via a direct current link. The alternating current networks each have a fundamental frequency, the nominal value of which is usually equal to 50 or 60 Hz.
During certain circumstances, oscillations of a frequency equal to the fundamental frequency can be initiated in the direct current transferred in the direct current link. In particular, the phenomena might occur when the plant exhibits on one hand a resonance on the alternating current side of the converter at a frequency equal to twice the fundamental frequency, and on the other hand a resonance on the direct current side at a frequency equal to the fundamental frequency, such as a combination of low impedance at the fundamental frequency as seen from the converter to the direct current link and a high impedance at twice the fundamental frequency as seen from the converter to the alternating current network. This phenomena might give rise to direct currents through the secondary windings of the converter transformer with an amplitude sufficient to bring the magnetic circuit of the transformer into saturation. The oscillations might build up comparatively slowly, during periods of the order of minutes and even hours.
The normal current control of the converter is not always able to sufficiently suppress this type of oscillation, which has as consequence that it will be sustained until the protective system of the plant is activated and trips the plant.
In the European Patent EP 0 671 067 a method and an arrangement for damping oscillations at or near a natural resonance frequency in a power transmission system are described. An oscillator is adapted to generate reference signals of the form sin .omega..sub.r t and cos .omega..sub.r t where .omega..sub.r is a pre-set frequency, selected with knowledge of a natural resonance frequency of the power transmission system, a frequency which can be equal to the fundamental frequency or to another frequency, for example a sub-synchronous frequency.
A measured value of a quantity sensed in the power transmission system, exemplified as the direct current in an HVDC transmission svstem, is multiplied with the reference signals. The products obtained are of the form X sin .PHI. and X cos .PHI., respectively, where X is the amplitude of the measured value, and .PHI. the phase position thereof in relation to a reference phase position. Each product is treated in a separate calculating circuit to the effect to add to the phase position .PHI. an adjustable phase angle displacement .alpha., is multiplied with the reference signals cos .omega..sub.r t and sin .omega..sub.r t , respectively, and thereafter added. The sum is multiplied with an amplification constant K. The result is a control signal of the form K cos (.omega..sub.r t+a+.PHI.), which control signal is supplied to the power transmission system, in the case this is an HVDC plant, for example in such a way that the ordered control angle is generated in direct dependence on the control signal. The control system operates as a sampled system and the phase angle displacement .alpha. is chosen for best damping with respect to delays in the sampling.
The products X sin .PHI. and X cos .PHI. are supplied to a diagnostic and determination circuit, wherein each of them is compared to a predetermined limiting value. If anyone of the limit values is exceeded, the control signal is supplied to the power transmission system in dependence on a decision circuit comprising delay circuits with pre-selected time delays. The criteria for the supply of the control signal to the power transmission system is that a limiting value is exceeded during a certain period of time, for example 15 ms.
The generation of the products X sin .PHI. and X cos .PHI., respectively, can be interpreted as a projection of the measured value on two orthogonal axes in a co-ordinate system which rotates with an angular velocity corresponding to the pre-set frequency .omega..sub.r of the oscillator, and the measured value is thus represented by a vector in this co-ordinate system. For the case that the frequency of the oscillation of the measured value coincides with the frequency of the oscillator, this vector is stationary in the co-ordinate system, that is, its projections on the axes are constant in time. For the case that the frequency of the oscillation of the measured value is different from the frequency of the oscillator, the vector rotates in the co-ordinate system with an angular velocity corresponding to the difference between the frequency of the oscillator and that of the measured value. In this case, the projections of the measured value will fluctuate in time. If the limiting values are symmetrical with respect to positive and negative deviations, the lines connecting the in the co-ordinate system plotted limiting values will form a rectangle with its centre at the origin.
As is understood from the above, the mentioned delay of the supply of the control signal, when a limiting value is exceeded, implies that with the in the patent proposed method also an oscillation of a frequency which differs from the frequency of the oscillator will be detected.
Thus, this method allows for detection of oscillations with frequencies within a certain range around the pre-set frequency of the oscillator, that is, the detection system has a certain band width, whereas the counteracting damping action initiated via the control signal is of the pre-set frequency.
The fundamental frequency of an alternating current network is, however, in general not constant but varies in dependence on various operating parameters. In strong networks with good control of the frequency, the frequency deviations are typically .+-.0.1 Hz, in extreme cases they can amount to the order of .+-.5 Hz. For the oscillation phenomena described in the introductory part of the description, the frequency of the oscillation is equal to the actual value of the fundamental frequency, whereas an oscillator as proposed in the patent should then be pre-set to oscillate with the nominal value of the fundamental frequency.
With a counteracting action at the nominal value of the fundamental frequency, there will be a risk that, with the gain required to counteract an oscillation of the kind described in the introductory part of the description, such a control signal would interfere with and influence also other phenomena and processes in the plant, for example in such a way that the mentioned saturation of the converter transformers would be circulating between the phases of the transformer with a certain beat frequency.